feat: tree lines, scanner improvements, themes, tooltips, README overhaul

- Tree line connectors (Unicode box-drawing ├─ └─ │) at arbitrary depth
- Fix editor overwriting tree chars at depth 2+ (applyMarginText Pass 2)
- Scanner: unknown value scan, comparison rescan modes (Changed/Unchanged/Increased/Decreased)
- New Tailwind theme (tw.json), WCAG contrast fixes for warm/mid themes
- Tooltip system (rcxtooltip.h)
- Comprehensive README rewrite with full feature inventory
- New tests for compose tree lines, scanner, tooltips

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/mcp/mcp_bridge.cpp

@@ -1214,9 +1214,13 @@ QJsonObject McpBridge::toolUiAction(const QJsonObject& args) {
     }
 
     if (action == "reset_tracking") {
-        if (!ctrl) return makeTextResult("No active tab", true);
-        ctrl->resetChangeTracking();
-        return makeTextResult("Value tracking reset. All histories cleared.");
+        int count = m_mainWindow->tabCount();
+        for (int i = 0; i < count; ++i) {
+            auto* t = m_mainWindow->tabByIndex(i);
+            if (t && t->ctrl)
+                t->ctrl->resetChangeTracking();
+        }
+        return makeTextResult(QStringLiteral("Value tracking reset on all %1 tabs.").arg(count));
     }
 
     return makeTextResult("Unknown action: " + action, true);